The privacy of the voting booth offers substantial protections to a voter. You might threaten or bribe me, but I can vote as I please, because you can't ever know how I actually voted. Many of these protections disappear if your ballot paper is delivered to your home and returned by post.
Husbands can coerce their wives into registering for a postal ballot and then vote on their behalf. Religious leaders can coerce their followers into registering for a postal ballot and handing it over to a chosen candidate. A corrupt care home worker could register dozens of residents for postal voting, then sell their ballot papers. Migrants with a poor grasp of the English language can be tricked into giving up their ballot paper. Poor people can be offered cash for their ballot paper. Postal ballots can be stolen from postboxes and sorting offices.
All of these frauds have taken place in the UK; local election results have been nullified on more than one occasion due to endemic fraud.
While my intuition would be that incidents are fairly rare for social reasons, intimidation/coercion with postal ballots is essentially invisible, until and unless people come forward and complain (which intimidation and coercion itself can be deployed against.) So, even if it was common, the elections would look smooth by the things we usually look at.
It's also a precaution against future regressions: you want to design protections like this for the worst political climates, not the best, so you'll be ready if e.g. an employer starts thinking they can get away with abuse during a recession or a union leader decides desparate times warrant desparate measures.
It's much easier to prevent that kind of thing rather than repair the damage after the fact, similar to the problems we've seen with gerrymandering.